I wanted to bring this preamplifier to the attention of folks in this forum to show how well it can work as a headphone amp, when modified to have higher bias current and lower voltage rail. Originally designed to drive high impedance loads up to 40Vpp, a few resistor tweaks and swapping output device from KSA1381 to 2SA1837 (n.b., pinoutss are flipped), increasing bias from 17mA to 130mA, and dropping voltage from 48V to 24V. But all in same basic package. This is a SE Class A amp with a modified LTP input stage. No fancy components, no matching needed, just nice performance and great sound. Very easy to build. Thanks to Aksa (Hugh Dean) for circuit design, and JPs64 for superb professional layout work.

they were only about $1.60ea and yes, from Aliexpress. I literally used them to give a nice racy look to a bland looking green PCB!

They sound fine and as good as an equivalent part from a European brand. All my other caps are real Nichicons, Pansonic, Wima (except for the 100uF 63v bias smoother on the daughterboard - those are from Ali but measure exceptionally well on LCR meter and I have a bag of 100 of them).

My listening tests with critical high resolution test tracks with 192kHz FLAC show that this amp with all of its coupling caps, is just as resolving, if not even more so than a DC coupled Schiit M3, but with superior soundstage and transparency.

So I say, for $1.60, it's harmless and go ahead and buy them and try them. Another good source of nice MKP tube shaped audio caps is Dayton speaker crossover caps. About $3 for the same thing but in 2.2uF and 250v and 1%. I'm sure they are from China but don't say so on the cap.

they were only about $1.60ea and yes, from Aliexpress. I literally used them to give a nice racy look to a bland looking green PCB!

They sound fine and as good as an equivalent part from a European brand. All my other caps are real Nichicons, Pansonic, Wima (except for the 100uF 63v bias smoother on the daughterboard - those are from Ali but measure exceptionally well on LCR meter and I have a bag of 100 of them).

My listening tests with critical high resolution test tracks with 192kHz FLAC show that this amp with all of its coupling caps, is just as resolving, if not even more so than a DC coupled Schiit M3, but with superior soundstage and transparency.

So I say, for $1.60, it's harmless and go ahead and buy them and try them. Another good source of nice MKP tube shaped audio caps is Dayton speaker crossover caps. About $3 for the same thing but in 2.2uF and 250v and 1%. I'm sure they are from China but don't say so on the cap.

Amazing to see how versatile this circuit/board can be! Which version shall I build now ???
I also love your 'hydraulic' pot knob !
Cheers,

Jacques

Well, the nice thing is with this flexible design is you don't need to make a whole new board for each amp. Just make a second set of daughterboards with the revised parts and have on hand for a headphone amp. You could also, just use the HPA as a preamp if you don't need to large voltage swing. The daughterboards and the components are very inexpensive to make.

Another variant is to run Vcc=44V and 71mA bias and that would work as both a high swinging preamp and a headphone amp suitable for 120ohms or greater (or high sensitivity IEMs) headphone amp. Jack of all trades. Dissipation on outputs would be 1.56w/ea the same as 130mA at 24V.

You might want to mention in this thread that the pin configuration for the 2SA1837 is different to the KSA1381 transistor.

Good point, but I figured folks would realize that when they seen that the 2SA1837 mounts on the backside of the board - which is a good thing too for two reasons: there is not enough room on one side for two heatsinks, and secondly, the mass distribution is balanced so the card doesn't lean

I had a few more hours of critical listening last night and played with the input caps. I removed the 4.7uF Wima and left the ChiFi 1.0uF MKP. The bass extension was reduced by -0.5dB at 20Hz and 1.3dB at 10Hz, as predicted. But this is a fine value given that there may be improved claroity and resolution from using just one input cap.